Introduction :
Light is the form of energy which makes object visible. It means that we can see an object only when light from an object enters our eyes. The light may have been emitted by the object or may have been reflected by it. You may have also seen beams of light from the torch, head light of vehicles. 

Sources of Light :
Those bodies which emit light in all directions are called sources of light. The sources can be point one or extended one. The sources of light are of two types :

(i) Luminous sources : Those objects which by itself emit light are called luminous sources. 

Eg. : Sun and stars (natural luminous sources), electric lamps, candles and lanterns (artificial 
luminous sources). 

(ii) Non-luminous sources : Those objects which do not emit light but become visible only when light from luminous objects falls on them. They are called non-luminous sources. 

Eg. : Moon, planets (natural non- luminous sources), wood, table (artificial non-luminous sources) etc.

(iii) Bioluminescence : The production or emission of light by a living organism as a result of some chemical energy getting converted into light energy is called bioluminescence. The name originates from the Greek word bios for ‘living  and the Latin word lumen for ‘light’.
Fireflies are insects that give off a pale, greenish-yellow light that flashes or glows in the dark . Some of the fish, such as angler fish, living deep under the sea are also bioluminescent.

Medium of light :

Substance through which light propagates or tends to propagate is called a medium of light.
According to the medium of light objects are divided into three parts :

(i)     Transparent object :

Bodies that allow light to pass through them i.e. transmit light through them, are called transparent bodies. 

 Ex. : Glass, water, air etc.

(ii)     Translucent object :  Bodies that can transmit only a part of light through them are called translucent objects. 

 Ex. : Froasted or ground glass, greased paper, paraffin wax etc.

(iii)   Opaque object : Bodies that do not allow light to pass through them at all are said to be opaque objects.

Ex. : Chair, desk etc.

Some common characteristics of light are given below:
(i)     Light is invisible form of energy .
(ii)   Light does not require material medium for its propagation i.e. light can travel through vacuum.
(iii)   The speed of light in free space (vacuum) is  3 × 108 m/s. Its speed is marginally less in air. Its speed decreases considerably in glass or water.
(iv)  Light undergoes reflection from polished surfaces such as mirrors etc.
(v)   When light goes from one medium to another, it changes its path.This phenomena is called refraction.

Some definition related to the light :
(A)     Ray of Light :     
The path along which light energy travels in a given direction is called ray of light. A  ray of light  is represented as a straight line. The arrow head on it gives the direction of light.

(B)     Beam of Light : 
A collection of rays of light is called beam of light. However, if the number of rays is too small then such a collection of rays is called Pencil of light.

(C)     Parallel Beam : 
When the rays of light travel parallel to each other, then the collection of such rays is called parallel beam of light. For example, sun rays entering into a room through a ventilator constitute a parallel beam .